Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
This invention relates generally to hydrophones, and more particularly to a
pressure compensated fiber optic hydrophone.
BACKGROUND OF THE INVENTION
Fiber optic hydrophones are well known in the art for measuring seismic and
acoustic disturbances. Generally hydrophones are towed behind a ship to
measure these
disturbances. However, with the increasing development of subsea or land-based
oil/well
systems, a hydrophone that could be deployed down a well at extreme depths and
that
could withstand the extremely corrosive downhole environment would provide
significant
benefits. Such a hydrophone would improve the ability to explore the land
surrounding a
well site by seismology or to detect other acoustics downhole that could
inform the well
operator about various aspect of the well's production.
While hydrostatic pressure has a measurable effect on a hydrophone, especially
when the hydrophone is deployed at extreme depths, small dynamic pressures,
such as
propagating acoustic sound waves, have a relatively small effect and therefore
are more
difficult to measure. When a measurement is to be made at depths where the
hydrostatic
pressure is great (e.g., thousands of feet down the well), the hydrostatic
pressure can
overwhelm the acoustic waves by many orders of magnitude.
In an attempt to resolve relatively small dynamic pressures, fiber optic
hydrophones generally have two fiber optic "arms"-a sensing arm and a
reference arm.
Both the sensing arm and the reference arm generally constitute optical fibers
coiled
around corresponding cylindrical mandrels-an outer compliant mandrel for the
sensing
arm and an inner rigid mandrel for the reference arm. The compliant mandrel is
typically
thin walled so that its radius changes easily in response to the acoustic
pressures being
measured. A cavity is formed between the two mandrels. A gas (e.g., air) or
liquid
typically fills this cavity. The rigid mandrel may be relatively thick walled,
or
alternatively thin walled and exposed to the ambient pressure so that its
radius would not
change. One such hydrophone is disclosed in U.S. Patent 5,394,377 entitled,
"Polarization
Insensitive Hydrophone". While compliant mandrels are very sensitive, they are
subject
to damage and collapse when subjected to extremely high hydrostatic pressures,
particularly if they are gas-backed. The production of such gas-backed designs
is also
costly, largely due to the need to seal the air
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cavity existing between the sensing and reference mandrels. Furthermore, the
reference
fiber must enter and exit this air cavity without disrupting the seal. Leaking
and fiber
breakage at this seal commonly can occur during the assembly process.
An alternative design that attempts to alleviate the problems with gas-backed
designs comprises a solid core wrapped with a reference coil of optical fiber.
A compliant
material is formed around the reference coil such that a cavity is eliminated.
Then a
sensing coil of optical fiber is wound around the compliant material. Such a
design is
disclosed in U.S. Patent 5,625,724 entitled, "Fiber Optic Hydrophone Having
Rigid
Mandrel". While this solid design withstands high pressures when deployed at
extreme
depths, the design lacks in sensitivity to detect acoustic pressure waves and
requires two
windings of optical fibers. Other fiber optic hydrophone designs can be found
in U.S.
Patents 5,625,724; 5,317,544; 5,668,779; 5,363,342; 5,394,377.
The art would benefit from a hydrophone sensitive enough to measure relatively
small dynamic pressures while being able to withstand deployment in
environments
having large hydrostatic pressures. It would be further beneficial for such a
hydrophone to
contain a single measurement coil, without the need for a reference coil.
SUMMARY OF THE INVENTION
A pressure compensated hydrophone for measuring dynamic pressures is
disclosed. The hydrophone includes a compliant hollow mandrel with a single
optical
fiber coiled around at least a portion of the mandrel. The mandrel further
includes at least
one pressure relief valve for compensating for changes in hydrostatic
pressure. The
pressure relief valve includes a micro-hole that allows hydrostatic pressures
or low
frequency pressure events to couple into the interior of the mandrel to
provide
compensation against such pressure. Higher frequency pressure events of
interest do not
couple through the micro-hole and therefore act only on the exterior of the
mandrel,
allowing for their detection. Because (quasi) hydrostatic events are
compensated for, the
mandrel may be made particularly compliant, rendering the singular fiber optic
coil
particularly sensitive to the detection of the higher frequency signals of
interest.
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BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other features and aspects of the present disclosure will be
best
understood with reference to the following detailed description of embodiments
of the
invention, when read in conjunction with the accompanying drawings, wherein:
Figure 1 illustrates a cross sectional view of one embodiment of a pressure
compensated hydrophone incorporating a single pressure relief valve.
Figure 2 illustrates a cross sectional view of one embodiment of a pressure
compensated hydrophone incorporating first and second pressure relief valves.
Figure 3 illustrates a perspective view of an embodiment of a pressure
compensated hydrophone.
Figure 4 illustrates a perspective view of another embodiment of a pressure
compensated hydrophone.
Figure 5 illustrates a cross sectional view of one embodiment of a pressure
compensated hydrophone package assembly.
Figure 6 schematically illustrates an array of hydrophone package assemblies
deployed in a well and connected by inter-station cables.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENT'ION
In the interest of clarity, not all features of actual implementations of a
pressure
compensated hydrophone are described in the disclosure that follows. It will
of course be
appreciated that in the development of any such actual implementation, as in
any such
project, numerous engineering and design decisions must be made to achieve the
developers' specific goals, e.g., compliance with mechanical and business
related
constraints, which will vary from one implementation to another. While
attention must
necessarily be paid to proper engineering and design practices for the
environment in
question, it should be appreciated that the development of a pressure
compensated
hydrophone would nevertheless be a routine undertaking for those of skill in
the Eu-t given
the details provided by this disclosure.
Figure 1 depicts an embodiment of a pressure compensated hydrophone 10. The
3o hydrophone 10 includes a preferably flattened oblique mandrel 24 (shown
best in Fig. 4)
that contains a pressure-relief valve 12 and an inner cavity 32. The inner
cavity 32 spans a
portion of the length of the mandrel 24 and is preferably filled with a high-
viscosity low
bulk modulus fluid, such as silicone oil, such that substantially no air is
present within the
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inner cavity 32. The inner cavity 32 acts in tandem with the pressure relief
valve 12 to
provide pressure compensation for the hydrophone 10, as described in more
detail below.
The inner cavity 32 is bounded by a wall 25 to define a sensing region 33 of
the
hydrophone 10. The hydrophone can range from 0.4 to 12 inches in length and
from 0.4
to 1.5 inches in diameter, depending on the application at hand.
The mandrel 24 is preferably made of a homogenous material, which will impart
a
compliance to wall 25 suitable for the particular application at hand. Metal
alloys
providing suitable compliancy and chemical robustness for oil/gas well
applications
include non-ferrous alloy materials, alloy steel, or stainless steel. The
compliance may
vary depending on factors such as the thickness of the mandrel wall 25, and
the physical
properties of the mandrel material, e.g. its modulus of elasticity. These
factors and others
may be chosen to help produce favorable sensor sensitivity for detecting the
frequencies
and magnitudes of interest, as one skilled in the art will realize. For an
oil/gas well
application, it is preferred that the wall 25 be from 0.005 to 0.1 inches
thick, and that the
sensing region 33 be from 0.1 to 10 inches long. Different materials or pieces
could be
used for the mandre124 and the wall 25, although it is preferred that they be
integral. The
mandre124 may be formed by standard metal working processes, pressing methods,
or an
extrusion or drawing process.
A standard optical fiber 26 is coiled around the outside of the mandrel 24
under a
2o predetermined amount of tension and along at least a portion of the sensing
region 33.
This coil 55 is preferably secured in place around the sensing region 33 by
covering it
with, an epoxy, adhesive, encapsulating or potting compound, or any other
securing means
(not shown) capable of withstanding environment (e.g., temperature) into which
the
mandrel will be deployed. When the hydrophone 10 is subjected to a pressure,
e.g., Po,
that pressure will exert a force perpendicular to the sensing region as shown.
Thus, in the
sensing region 33, the pressure will compress the mandrel 24 inward causing
the wall 25
of the mandrel 24 to deform. When the mandrel 24 deforms, the coil 55 of
optical fiber 26
will correspondingly change in length. Optical detection of this change in
length thus
allows a determination of the pressure, Po, as will be described in more
detail below.
The sensitivity of a fiber optic hydrophone using interferometry principles is
a
function of the change of strain of the fiber optic coil 55. As noted
previously, the coil 55
is preferably pre-strained, or tension wound, such that when the wall 25 of
the mandrel 24
deforms inward, the coil 55 will still maintain intimate contact with the wall
25.
Maintaining such contact thus helps to maximize the sensitivity of the coil
and increases
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the magnitude of pressures that may be detected. The other objective of
pretension is to
keep the sensing fiber always in tension and not operating in the
compressional mode.
Coil sensitivity is further affected by the number of turns in the coil 55. As
the mandrel
deforms, each turn of the coil 55 will change in length by a slight amount,
but this amount
is amplified, and therefore easier to optically resolve, when more turns are
used. In short,
increasing the number of turns will generally increase the sensitivity of the
coil 55. While
an appropriate length will necessarily depend on the application in question,
coil lengths
of 5 to 300 feet are believed preferable for detection of downhole acoustics.
The coil 55
can consist of a single layer or multiple stacked layer of optical fiber 26
depending on the
application.
The mandrel 24 may further include pre-drilled holes 49, 53 to aid in its
attachment
to another body as described in more detail below. As shown in Figure 1 the
mandrel 24
is formed around a discretely formed pressure relief valve 12, although the
mandrel and
the housing of the valve may be formed as one integrated unit.
Preferably, the fiber 26 further includes fiber Bragg gratings (FBGs) 27a, 27b
adjacent' to both ends of the coil 55. Light reflected from the FBGs 27a, 27b
provides
information about the length of the optical fiber, and hence the pressure of
the detected
acoustics, between the two FBGs. If the FBGs have the same reflection
wavelength, the
reflected signals will form an interference pattern that can be resolved using
fringe
counting techniques or other demodulation techniques. One method for
interrogating a
coil using an interferometric approach is disclosed in U.S. Patent No.
6,785,004, entitled
"Method and Apparatus for Interrogating Fiber optic Sensors," filed November
29, 2000.
It should be noted that the use of FBGs bounding the coil 55 is not strictly
necessary. If the hydrophone 10 does not contain FBGs, other known
interferometric
techniques may be used to determine the change in length (circumferential or
axial) of the
coil 55, such as by Mach Zehnder or Michaelson interferometric techniques,
which are
disclosed in U.S. Patent 5,218,197, entitled "Method and Apparatus for the Non-
invasive
Measurement of Pressure Inside Pipes Using a Fiber Optic Interferometer
Sensor," issued
to Carroll. The coils may be multiplexed in a manner similar to that described
in
Dandridge et al., "Fiber Optic Sensors for Navy Applications," IEEE, Feb.
1991, or
Dandridge et al., "Multiplexed Interferometric Fiber Sensor Arrays," SPIE,
Vol. 1586,
1991, pp. 176-183.
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Alternatively, the FBGs may have different reflection wavelengths in a
Wavelength Division Multiplexing (WDM) approach. Moreover, the FBGs
themselves,
instead of the coil 55 between them, can be coiled around the sensor and used
as the
sensor(s) for the hydrophone. In such an embodiment, the deformation of the
wall 25
would manifest as shifts in the reflection wavelengths of the FBGs, which
could be
correlated to the pressures being detected, as is well known and not further
discussed. In
the preferred embodiment of Figure 1, the FBGs 27a, 27b are located so as to
experience
little to no strain, as strain on the FBGs will shift the wavelength of light
reflected
therefrom which might disturb the pressure measurement. Thus, the optical
fiber
preferably lies along the mandrel 24 at least slightly outside of the sensing
region 33 and
compliant wall 25. Alternatively, the FBGs 27a, 27b may be isolated from the
wall 25 by
isolation pads or similar devices, as is disclosed in U.S. Patent No.
6,784,004, entitled
"Apparatus For Protecting Sensing Devices," filed November 29, 2000.
As alluded to earlier, the disclosed hydrophone further includes a pressure
relief
valve 12 to compensate for changes in hydrostatic pressure, which may result
as the
hydrophone is deployed deeper and deeper into a well. The pressure relief
valve 12
preferably includes a micro-hole 14. This micro-hole 14 acts as a mechanical
low ~pass
filter that has a diameter such that pressure waves above a certain frequency,
e.g., 3Ez,
are unable to pass through the micro-hole 14. Because these higher frequencies
will not
exert a pressure on the valve 12, they will not affect the pressure inside the
inner cavity 32,
which allows the presence of such higher frequency components to be detected
by the coil
55. By contrast, frequencies below this cut off will exert pressure both
inside and outside
of the coil, and will not be detectable. As most frequencies of interest in
acoustic
phenomenon to be detected are above this range, this frequency limitation does
not
appreciably limit the operation of the hydrophone. In a preferred embodiment,
the
diameter of micro-hole 14 ranges from about 0.001 to 0.1 inches.
The micro-hole 14 in conjunction with the valve 12 allows for the compensation
of
hydrostatic pressures. The valve 12 includes a housing 23 containing a ball 18
normally
biased against an elastomeric 0-ring 22 by a spring 16. The spring 16 exerts a
predetermined force against the ball 18 ("valve closing force"), which is
determined by the
amount of compression of the spring and its spring constant. Preferably, this
force
maintains approximately a 50psi difference between the PI of the inner cavity
32 and the
Po of the outer environment. In one embodiment, the valve 12 may comprise a
0.187"
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Unscreened Pressure Relief Valve manufactured by The Lee Company. This valve
is
constructed entirely of stainless steel, has a diameter of 3/16 inch, is
approximately 1/2
inch long, and imparts a valve closing force from 20 to 100 psi.
As the components of the valve 12 may become exposed to the fluids present in
the
well, it is preferred that they be made of suitably resilient materials. Ball
18 may be made
of a metal alloy such as stainless steel, ceramic, or plastic or rubber
materials such as
closed cell synthetic rubber, solid natural rubber, polyurethane,
polyethylene, silicone
rubber, or neoprene. The ball 18 may be hollow and may take other shapes
(e.g.,
cylindrical) so long as it is movable in response to the increasing external
pressure and is
capable of forming a good seal. If the ball is made of a deformable material,
the 0-ring 22
may be eliminated from the pressure relief valve 12. The spring 16 preferably
comprises a
metal alloy such as stainless steel. Biasing means other than springs may also
be used so
long as they are sufficient to maintain the required internal pressure P1
within the inner
cavity.
It is preferred to form the valve 12 within its housing 23 before coupling the
housing 23 to the mandrel 24, although these components can be formed as an
integral
piece. Coupling between the housing 23 and the mandrel 24 may be effectuated
by a
screw relationship, by welding, or by other well known means (not shown).
Thereafter,
the inner cavity 32 of the hydrophone can be filled with oil by using a thin
probe to
depress the ball and introducing oil through the micro-hole 14. Alternatively,
the inner
cavity 32 can be filled with oil prior to the coupling of the housing 23 to
the mandrel 24.
As noted earlier, some prior art hydrophones were limited with respect to the
pressures to which they could be exposed, as high pressures presented the risk
of
collapsing the relatively thin wall around which the sensing coils were
wrapped. This
problem has been alleviated in the disclosed hydrophone design because the
pressure
inside of the hydrophone can roughly be brought into equilibrium with the
external
hydrostatic pressure. When the external pressure Po exceeds the valve closing
force of
valve 12 (e.g., 50 psi), the ball 18 of the valve 12 will start to open, which
allows the
external pressure to couple into the inner cavity through micro-hole 14.
(Depending on
the viscosity of the oil in the inner cavity 32 and the diameter of the ball
18 within its
housing 23, the well fluid and the oil within the hydrophone may mix, but this
is not
deleterious to the operation of'the hydrophone. Should particulates in the
well fluid cause
concern that the valve might become jammed, a mesh or screen (not shown) may
be
placed within the micro-hole 14). Accordingly, the hydrophone 10 may be
deployed to
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great depths and subjected to great pressures (e.g., 20,000 psi) while still
retaining a
relatively thin (and dynamically sensitive) wall 25, which is capable of
detecting higher
frequency acoustic phenomenon as explained earlier.
Figure 2 discloses an embodiment of the hydrophone which provides both
descending and ascending pressure compensation, and which incorporates two
pressure
relief valves 12a, 12b. Valve 12a allows for descending pressure
compensat:ion, as
described above. Valve 12b, which is similar (or identical) in structure to
valve 12a,
allows for ascending pressure compensation, and operates as follows. When the
hydrophone 10 is raised from a lower depth to a higher depth, the external
hydrostatic
pressure decreases. Because the inner cavity had been coupled to a higher
pressure at the
lower depth, the volume of the fluid within the inner cavity 32 will expand at
the higher
depth. When the pressure of the inner cavity 32 exceeds the sum of the
external pressure
and the valve closing force of valve 12b (again, preferably 50 psi), valve 12b
will open
and equilibrate the external and internal pressures. When the external
pressures fall below
the valve closing force (e.g., 50 psi), valve 12b will close, thus trapping
the fluid within
the inner cavity 32 at the valve closing force. One skilled in the art will
recognize that
valves 12a and 12b are "one way" valves. Accordingly, when the hydrophone
descends,
valve 12b is prevented from opening due to the pressure the ball 18b exerts on
the 0-ring
22b; similarly, when the hydrophone ascends, valve 12a is prevented from
opening due to
the pressure the ball 18a exerts on the 0-ring 22a. In summary, the structural
integrity of
the hydrophone 10 as shown in Figure 2 remains intact as the hydrostatic
pressure
changes.
Depending on the application at hand, an embodiment of the disclosed
hydrophone
could have either or both of the valves 12a, 12b. For example, if it is not
anticipated that
the hydrophone 10 will be retrieved, valve 12b, providing for ascending
pressure relief,
may not be necessary. Moreover, if the hydrophone 10 is not going to be placed
sufficiently deeply such that descending pressure compensation will cause a
problem, or if
the inner cavity can be pre-pressurized to a suitably high value, then valve
12a, providing
for descending pressure relief, may not be necessary. Additionally, in an
embodiment
having both valves 12a, 12b, the valve closing forces of the two valves need
not be the
same.
The disclosed hydrophone 10 may be cylindrical in shape as shown perspectively
in Figure 3, but may also comprise a preferable more flattened shape as shown
in Figure 4.
This flattened, oblique cylindrical, shape renders the hydrophone more
sensitive to the
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dynamic acoustic pressures being measured, as the hydrophone is more compliant
along
the elongated surfaces when compared with a cylindrical embodiment.
Figure 5 discloses the hydrophone 10 within a perforated housing 34 to form a
hydrophone package assembly 20. Essentially, housing 34 provides mechanical
protection
to the hydrophone 10 (and particularly to the fiber optics), while still
allowing dynamic
and static pressures to couple to the hydrophone 10 through holes 75. The
housing 34 may
include a first recessed end 76 and a second open end 77. The first recessed
end 76 of the
housing 34 is joined to a disc 35. The disc 35 and the housing 34 are composed
of a metal
suitable for the intended environment of the hydrophone assembly 20, such as
stainless
steel or Inconel. The disc 35, the housing 34, or both further include
pressure relief holes
75 for allowing the well bore fluid to enter into the housing cavity 42.
Preferably the fiber
26 is sufficiently encapsulated with a coating material, such as an epoxy, to
protect the
fiber 26 from the corrosive effects of the well bore fluid. The thickness of
the disc 35 or
housing 34 may be varied depending on the temperature and harshness of the
environment
and the expected pressure. The disc 35 is preferably joined to the recessed
end 76 of the
housing 34 by laser welding, although other techniques or methods known in the
art can
be used. Furthermore, the disc 35 and the housing 34 may be formed into one
integral
housing or sleeve as opposed,to joining two separate pieces together. The
second end 77
of the housing 34 is joined to an end cap 46, which further includes an
optical feedthrough
38 such as disclosed in U.S. Patent No. 6,526,212, entitled "Optical Fiber
Bulkhead
Feedthrough Assembly And Method For Making Same," filed on July 28, 2000. The
fiber
optic feedthrough 38 allows the fiber 26 to pass through the end cap 46 on its
way to the
optical source/detection equipment preferably residing at the surface of the
well (not
shown). A metal capillary tube 44, or series of interconnecting tubes,
preferably protects
the fiber 26 as it exits the housing 34. The capillary tube(s) 44 is
preferably welded to the
end cap 46, and details concerning the welding process and other applicable
manufacturing details are disclosed in U.S. Patent No. 6,888,972, entitled
"Multiple
Component Sensor Mechanism," filed October 6, 2002. The feedthrough 38
preferably
seals the fiber 26 in place with an epoxy, glass, or other sealing material
known in the art
depending on the intended pressure and temperature to be encountered. The end
cap 46
may then be threadably connected to the housing 34 or may be connected by
other known
mechanical means or by welding. If the end cap 46 is welded to the housing 34,
the end
cap should have an end cap shoulder 57 that extends a
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sufficient distance within the inner dimension of the housing 34 to dissipate
heat during
the welding operation. For example, the shoulder 57 of the end cap 46 may
extend
approximately 4.5 mm into the housing 34, which has an inner dimension of
approximately 19 mm.
The hydrophone 10 is supported within the housing 34 preferably by the use of
locating pins 48 attached to the end cap 46, which may be similar to clevis
pins. The
locating pins 48 fit within pre-drilled holes 49 where a second smaller pin
52, such as or
similar to a cotter pin, is inserted into the locating pin 48 to lock the
locating pin 48 in
place. The hydrophone 10 may further include a second pre-drilled hole 53 for
the
placement of the smaller pin 52 (see Figure 2). The hydrophone 10 is thus
sufficiently
supported within the housing 34 without making contact thereto except at the
location of
the pin mechanisms. As one will realize, one or more pin/locating pin
mechanisms may
be employed, and the scope of the present invention is not limited to the
embodiments
shown. Additionally, the hydrophone 10 may be affixed within the housing 34 in
other
ways, as one skilled in the art will realize.
Alternatively, the housing cavity 42 may be sealed from the well bore fluid.
With
a solid housing 34 and a corrugated diaphragm (not shown), instead of a
perforated disc
35, the hydrophone 10 (and in particular the fiber optics) would be protected
from the
corrosive affects of the well bore fluid. In such an embodiment, the housing
cavity 42
may be filled with a fluid such as silicone fluid. To alleviate the thermal
expansion of the
fluid when the hydrophone assembly 20 is exposed to high temperatures, a
compensator
(not shown) is preferably disposed within the housing 34. The compensator has
a variable
volume responsive to the thermal expansion of the fluid. The compensator may
preferably
comprise a hollow bellow composed of metal. In an additional embodiment, the
hydrophone 10 may preferably be enclosed within compliant tubing, which
provides for
static pressure compensation as well as allows the dynamic acoustics to couple
into the
tubing. Such compliant tubing may be formed from polyurethane or other similar
plastic
material. Furthermore, the tubing may be fluid-filled or alternatively have a
solid core
filled with, for example, polyurethane foam or other suitable material.
The hydrophone assembly 20 allows for the coil 55 to sense dynamic acoustic
pressure waves. The hydrophone assembly 20 is designed to be deployed in the
well
annulus between the production pipe 54 (shown in Fig. 6) and the well casing
62 where it
will be subjected to high temperatures, pressures, and potentially caustic
chemicals or
mechanical damage by debris within the annulus. Because these conditions could
CA 02461437 2007-12-19
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potentially damage an optical fiber, the pressure relief holes 75 may further
include a
mesh or filter device for preventing the entry of particles into the housing
cavity while
allowing the entry of static and dynamic pressures. The dynamic acoustics then
exert a
pressure onto the hydrophone 10 deforming the coil 55. The dynamic acoustics
may then
be detected, while the hydrostatic pressures are compensated for within the
hydrophone
cavity 32 as described previously. It should be noted however that the use of
a housing 34
is not strictly necessary, and the hydrophone could work in a given
environment without
such a housing. If a housing 34 is not used, the fiber optic cable and coil 55
should be
coated for protection, for example, with a suitably resilient epoxy as
mentioned earlier.
Turning to the schematic illustration in Figure 6, a fiber optic in-well
seismic array
68 used in the exploration of a hydrocarbon reservoir is depicted. The array
68 has a
plurality of seismic stations 60 which include the disclosed hydrophone
package
assemblies 20 interconnected by inter-station cables 56. The array 68 is shown
deployed
in a well 50, which has been drilled down to a subsurface production zone and
is equipped
for the production of petroleum effluents. Typically, the well 50 includes a
casing 62
coupled to the surrounding formations by injected cement. Production tubing 54
is
lowered into the cased well 50 with the seismic stations clamped thereto,
which may be
accomplished using the techniques and apparatuses disclosed in U.S. Patent No.
7,036.601, entitled "Apparatus and Method for Transporting, Deploying, and
Retrieving
Arrays Having Nodes Interconnected by Sections of Cable," filed October 6,
2002. The
well 50 can be fifteen to twenty thousand feet or more in depth.
The seismic stations 60 include hydrophone assemblies 20 and clamp mechanisms
64 such as disclosed in U.S. Patent No. 7,124,818, entitled "Clamp Mechanism
for In-
Well Seismic Sensor," filed October 6, 2002. The hydrophone assemblies 20 are
interconnected by the inter-station cables 56 to an instrumentation unit 70,
which may be
located at the surface or on an oil platform (not shown). The instrumentation
unit 70
typically includes optical source/detection equipment, such as a demodulator
and/or
optical signal processing equipment (not shown). The inter-station cables 56
(i.e., cable
44 of Figure 5) are typically 1/4 inch diameter cables housing optical fibers
between the
hydrophone assemblies 20 and the instrumentation unit 70.
The optical source within the instrumentation unit 70 may include a
semiconductor
laser diode that may be pulsed to effectuate the preferred interferometric
coil interrogation
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technique discussed earlier. However, and as one skilled in the art
understands, there are
various other optical signal analysis approaches that may be used to analyze
the reflected
signals from the hydrophone, such as (1) direct spectroscopy, (2) passive
optical filtering,
(3) tracking using a tunable filter, or (4) fiber laser tuning (if a portion
or all of the fiber
between a pair of FBGs is doped with a rare earth dopant). Examples of a
tunable laser
can be found in U.S. Patents 5,317,576; 5,513,913; and 5,564,832. One skilled
in the art
will also appreciate that the use of a fiber optic sensor in the disclosed
hydrophone easily
lends itself to multiplexing to other hydrophones or to other fiber optic
devices along a
single fiber optic transmission cable (i.e., cables 56), such as by the TDM or
WDM
approaches alluded to earlier.
The disclosed hydrophone assembly 20 has many potential downhole uses, but is
believed to be particularly useful in vertical seismic profiling to determine
the location of
petroleum effluents in the geologic strata surrounding the well in which the
hydrophones
are deployed. (Further details concerning vertical seismic profiling are
disclosed in U.S.
Patent No. 6,601,671, entitled "Method and Apparatus for Seismically Surveying
an Earth
Formation in Relation to a Borehole," filed July 10, 2000. As is known, a
seismic
generator (not shown) detonated at the surface near the well is used to
generate acoustic
waves which reflect off of the various strata and are detected by the
hydrophone
assemblies 20 at each seismic station 56. In this application, the seismic
stations 60 are
distributed over a known length, for example, 5000 feet. Over the known
length, the
seismic stations 60 can be evenly spaced at desired intervals, such as every
10 to 50 feet,
as is necessary to provide a desired resolution. Accordingly, the fiber optic
in-well
seismic array 68 can include hundreds of hydrophone assemblies 20 and
associated clamp
mechanisms 64. Because fiber optic connectors on the inter-station cables 56
between the
hydrophone assemblies 20 can generate signal loss and back reflection of the
interrogating
signals, the use of such connectors is preferably minimized or eliminated in
the array.
Instead, it is preferred to splice together the various components along a
single fiber optic
cable, which minimizes signal loss. Such splicing may be performed in
accordance with
the techniques disclosed in U.S. Patent No. 6,888,972. If optical loss is
still too significant
along the entirety of the array even when splicing is used, different fiber
optic cables can
be used to interrogate different sections of the array, which requires inter-
station cable 56
to possibly carry multiple fiber optic cables.
12
